Sublimation, in chemistry, act of changing a solid into a gas, or a gas into a solid, without going through a liquid stage. Some substances when heated vaporize and become a gas, without melting into a liquid first. When the gas is cooled, the substance returns directly to a solid state. Examples of such substances are dry ice and iodine crystals. Sublimation is used to purify substances: When the solid is heated, only the substance that is pure becomes a gas—the impurities remain solid. When the impurities are removed the substance becomes refined.